1. Field of the Invention
This invention relates to a Ni--Ti--Pd superelastic alloy material which presents a small stress hysteresis, its manufacturing method and an orthodontic archwire made of this alloy material.
2. Description of the Prior Art
In general, a metal material loaded with a stress exceeding its elastic limit results in permanent deformation. However, a certain kind of alloy such as Ni--Ti alloy has a function to go back to its original shape after unloading, even if a stress is loaded to provide such an alloy with a strain close to 10% at a temperature exceeding a reverse Martensite transformation finish (Af) temperature (which will be hereinafter referred to as Af temperature) as shown in FIG. 1. Namely, the Ni--Ti alloy or the like has a superelastic function and is called a superelastic alloy.
Incidentally, FIG. 1 is a typical graph of the stress-strain curve for a superelastic alloy in loading and unloading. Referring to FIG. 1, 1 represents a difference between a loading stress P.sub.1 in the flat range from a to b and an unloading stress P.sub.2 in the flat range from c to d, and this difference is called a stress hysteresis.
The Ni--Ti alloy may be practically used for actuators, toys, pipe coupling or the like by taking advantage of its shape memory properties. In addition, the range of use of the Ni--Ti alloy by taking advantage of its superelastic properties has been recently increasing. Thus, the Ni--Ti alloy has been put into practical use in various fields by taking advantage of stress-strain characteristics similar to rubber. Specifically, the Ni--Ti alloy is frequently used for frames of glasses, wires of brassieres, orthodontic archwires, antennas for portable telephones or the like.
Further, an alloy having a composition adapted for such purposes has been manufactured by adding a small amount of a metal element such as Cr, Fe, Co, V, Mn and B to the Ni--Ti alloy in order to improve the workability and alloy characteristics.
In general, a superelastic alloy material is deformed due to a moderate stress in loading and takes advantage of its high force in unloading. Thus, it is preferable that the superelastic alloy material goes back to its original shape due to a stress which is as close to the stress in loading as possible. Namely, the superelastic alloy material preferably presents a small stress hysteresis. Further, it is necessary that a residual strain should be 0%, or be close to 0% in unloading.
FIG. 2 is a graph of the stress-strain curve for a Ni--Ti superelastic alloy. As is apparent from FIG. 2, a stress hysteresis (shown by 1 in FIG. 2) is as large as approximately 300 to 400 MPa, and therefore, there has been a limit in the use of such a Ni--Ti superelastic alloy.
On the other hand, a Ni--Ti--Cu alloy has been developed as a superelastic alloy which presents a small stress hysteresis. FIG. 3 is a graph of the stress-strain curve for the Ni--Ti--Cu superelastic alloy.
It has been found that the stress hysteresis of the Ni--Ti--Cu alloy is reduced as the amount of Cu in the alloy is increased, and that the stress hysteresis in a composition containing, by atomic percent, approximately 10% Cu is reduced down to 100 to 200 MPa and 20% Cu is reduced down to 40 MPa on a laboratory level (S. Miyazaki, I. Shiota, K. Otsuka and H.Tamura. Proc. of MRS Int'l. Mtg on Adv. Mats., Vol. 9 (1989) Page 153, Hiroshi Horikawa and Tatsuhiko Ueki, Advanced Materials '93. V/B: Shape Memory Materials and Hydrides, edited by K. Otsuka et al. Trans. Mat. Res. Soc. Jpn., Volume 18B Page 1113, U.S. Pat. No. 5,044,947). The Ni--Ti--Cu alloy having such a feature is mainly used for orthodontic archwires.
However, the hot workability of the Ni--Ti--Cu superelastic alloy is remarkably reduced as the amount of Cu in the alloy is increased. Thus, an alloy containing a large amount of Cu cannot be manufactured on a factory level. In addition, the stress hysteresis cannot be reduced to only about 160 MPa under the existing circumstances.
Accordingly, the development of a superelastic alloy material which is excellent in workability and presents an extremely small stress hysteresis has been needed. Incidentally, it is necessary for the superelastic alloy material that the residual stress in unloading as described above should show 0% or be close to 0%.